Critical region [Solving inequalities] ... rev.0

Thanks Mona, Richard:

The Mathcad cross product is totally correct. The Mathcad notation [operator] is not standard, there is no standard for that algebra, only "national standards" ! The matter about the area of the triangle is to construct the u, v vectors for the 1/2[|uxv|] . This work sheet is updated. A documenters like Wikipedia , not capable of showing the working construct ... don't wonder about others: no better [not to say worse]. Often, working maths are pretty rare and hard to find

Jean